1. Field of the Invention
The present invention relates to a fabricating method of multilayer ultrathin films. More specifically, the invention relates to a fabricating method of well-organized multilayer films using spin self-assembly method with a very short process time.
2. Description of the Prior Art
Ultrathin multilayer films have attracted much interest for their wide applications such as sensors, integrated optics, friction reducing coating, biological surface, light-emitting devices (LED), or surface orientation layers.
In particular, as electric, electronic, communication and computer industries progress, ultrathin materials with thickness in the range of 5 to 10 nm are expected to play an important part in developing up-to-date devices and many studies on ultrathin films are being carried out.
Such ultrathin films, which are usually prepared by coating an organic single-molecular thin layer onto the substrate, can be used for modifying characteristics of the substrate surface, for example, for promoting adhesive strength, preventing friction or corrosion, or modifying electronic or optical properties.
For sufficient introduction and expression of desired properties in substrates, it is basically required that the thin film is uniform in thickness and has a well-ordered orientation. If single monolayer coating cannot sufficiently change the property of the substrate surface to the desired degree, multilayer ultrathin coating of substrate is needed.
For the ultrathin film to be useful in various up-to-date applications, it is required that the thickness of the thin film can be controlled in the Angstrom scale and the thin film can also be formed between different materials.
For the preparation of ultrathin monolayer film, spin-coating process has been known. The method, in which coating agent is placed onto a substrate and then the substrate is rotated at a high speed, can form a uniform thin film in a short period of time and such thin films are, for example, utilized in the production of semiconductor chip.
Although the above spin coating process is useful for preparing a relatively thick single layer film with a thickness more than 10 nm, it is not appropriate for the ultrathin film with thickness less than 10 nm. Particularly, the thickness of the film layer is not easily controlled and the applicable solvent is limited because the process does not use attractive force between multilayers in a thin film and the solvent is required to be insoluble on the respective film layers, which make it difficult to prepare ultrathin films with multilayer structure.
The old and conventional process for preparing multilayer ultrathin films is the Langmuir-Blodgett (LB) technique. In the Langmuir-Blodgett (LB) technique, a single molecular film is formed on water surface and is transferred onto the surface of a solid substrate to form an ultrathin film, and this process is repeated to prepare multilayer ultrathin films.
However, the LB technique has problems in that the prepared films are unstable and the condition of the technique is quite complicated, and thus is inconvenient for the automation and the large-scale application.
For solving the problem of the above method, U.S. Pat. No. 5,208,111 discloses a method for ultrathin thin films or multilayer thin films by introducing a molecular self-assembly method based on the electrostatic force between layers.
In the method, the substrate substituted with ion or ionizable compound is immersed in a water soluble polyelectrolyte solution so that the polyions are adsorbed onto the substrate surface to form a single molecular film, as a result, the charge of the substrate surface is changed from anion to cation or vice versa.
In this method, the substrate coated with one polyelectrolyte is again immersed in an aqueous polelectrolyte with opposite charge.
FIG. 1 shows a prior art fabrication of a multilayered thin film by a molecular self-assembly, in which a silicate or glass substrate is modified with a positively charged material to convert the charge of the substrate into positive charge (1) and then in contact with a negatively charged material (2) (typically, polyelectrolyte) to form an ultrathin film. Repetition of this cycle produces a multilayer thin film consisting of alternating layers of polycations and polyanions.
The thickness and conformation of each polymer layer deposited are determined by physical and chemical characteristics of the depositing solution. For example, solutions with relatively high polyion concentrations or high ionic strengths favor the formation of thicker polyelectrolyte layer due to the increased diffusion rate and the structural change of polymer chain whereas very dilute solutions produce thinner monolayers on the substrate.
This approach can be used to manipulate a variety of different polyions including conjugated polyions. Further, the method is expanding its application, for example, for preparing a hybrid multilayer thin film in which organic materials are bound with various inorganic materials such as nano-sized gold particle (Adv. Mater., 1997, 9, 61), Fe3O4 (Appl. Phys. Lett., 1997, 71, 2265), montmorillonite-inorgnic material with platelet structure (Nano Lett., 2991, 1, 45; J. Am. Chem. Soc., 1997, 119, 6821), CdS, CdSe, CdTe and ZnS known as semiconductor and quantum particles (J. Phys. Chem. B. 1998, 102, 4096) and many related studies are currently in progress.
While the above method forms ultrathin film using electrostatic attraction of ion or ionizable compound, U.S. Pat. No. 5,518,767 discloses a technique where a ultrathin film is formed by the attraction between a conductive polymer containing partially positive charge and polyions with negative charge instead of ionization.
In addition, U.S. Pat. No. 5,536,573 discloses a technique, as an improvement of the above U.S. Pat. No. 5,518,767, for preparing multilayer thin film by electrostatic attraction or hydrogen bonding using a conductive polymers in which the charge capacity can be controlled depending on the amount of dopant and the type of polyanion, to be applied to small-size electronic devices, chemical or biological sensors, anti-corrosion coatings and anti-static coatings.
In the above described methods, however, the thin film formation is based on the adsorption by the spontaneous diffusion of coating materials through their interactions such as electrostatic attraction, hydrogen bonding, or chemical bonding, and relatively long adsorption time of 5 to 30 minutes should be allowed to obtain stable thin films with a specific orientation.
This leads to several problems such as the long preparation time of the multilayer thin films and the reduction of productivity. In addition, without thorough washing using a flow of pure solvent after the adsorption of a polyelectrolyte layer, the weakly adsorbed polyelectrolyte chains significantly increase the surface roughness of the multilayer films, yielding poor film quality. Consequently, optimum conditions for both adsorption and careful washing steps are required in order to prepare well-defined multilayer films.